The present invention relates to an optical recording method, an optical recording apparatus and an optical storage medium, in or from such a storage medium, data being recorded, erased or reproduced with irradiation of a light beam (for example, a laser beam).
Phase-change optical storage media are data-rewritable storage media, such as, recent CD-RW, DVD-RW and DVD-RAM. Especially, DVD-RW and DVD-RAM are used for recording and rewriting a large amount of data, such as video data. Phase-change optical storage media require excellent overwrite characteristics as well as recording characteristics.
Structures of and recording methods for conventional rewritable phase-change optical storage media are as follows:
Phase-change optical storage media have a structure in which at least a dielectric layer, a recording layer, another dielectric layer and a reflective layer are laminated in order on a substrate having a bottom surface to be irradiated with a laser beam carrying a recording or reproducing power, or an erasing power. In a phase-change optical storage medium having such a structure, recording pulses are applied (irradiated) onto a recording layer with a laser beam having a recording power, to melt and rapidly cool down the recording layer, thus forming amorphous recorded marks thereon. Reflectivity of the recorded marks lower than that of the crystalline-phase recording layer allows optical reading of the marks as recorded data. In erasing the recorded marks, a laser beam having a power (erasing power) smaller than the recording power is irradiated onto the recording layer to raise the temperature thereof to a temperature in the range from the crystallization temperature to the melting point to change the recording layer from the amorphous phase to the crystalline phase for erasing the recorded marks, thus overwriting being enabled.
Japanese Unexamined Patent Publication Nos. 2002-237089 and 2003-20065 propose an optical recording method or an optical storage medium in which reflectivity of un-recorded sections is made lower than that of recorded sections for excellent jitter and overwrite characteristics in high-speed recording. However, the inventors of the present invention found that the proposed optical recording method and optical storage medium cannot achieve sufficient overwrite characteristics (particularly characteristics at initial overwriting) at high linear velocity (for example, DVD×4 speed or higher).
Recently appeared are optical storage media applicable to multispeeds (for example, ×4 speed) to DVD×1 speed, which allow recording at linear velocity selected among the multispeeds. For such multispeed applications, Japanese Unexamined Patent Publication No. 2000-155945 proposes a recording method with a power ratio of recording power Pwl to erasing power Pel in low linear-velocity recording lower than that of recording power Pwh to erasing power Peh in high linear-velocity recording. However, this method increases the erasing power Peh in high linear-velocity recording, which makes sections to be crystalline amorphous, thus lowering recording characteristics at initial recording. The inventors of the present invention found higher crystallization speed in recording-film composition or otherwise to avoid amorphous phase tends to cause segregation of recording films due to high erasing power Peh, which lowers reflectivity over repeated recording of 1000 times or more to lower recording characteristics, thus not preferable for overwriting.
Japanese Unexamined Patent Publication No. 2002-92889 proposes a recording method to provide constant erasing power Pe irrespective of recording linear velocity or recording power. The inventors of the present invention, however, found this method cannot provide acceptable recording characteristics over a wide range (from low to high) of linear velocity, each linear velocity having its own optimum erasing-power level.
Japanese Unexamined Patent Publication No. 2001-297481 proposes a method with a relationship (Pe/Pw)ref>(Pe/Pw)H>(Pe/Pw)L for power ratios (Pe/Pw)L, (Pe/Pw)ref and (Pe/Pw)H at low linear velocity, medium linear velocity and high linear velocity, respectively. The inventors of the present invention, however, found this method cannot provide acceptable overwrite characteristics for several times of overwriting because of low erasing power at low linear velocity.
As discussed above, the conventional optical recording methods with lower reflectivity of un-recorded sections than that of the sections after recorded or erased and the conventional optical storage media that exhibit low reflectivity on un-recorded sections after initialized cannot provide sufficient jitter characteristics after overwriting in high linear-velocity recording, thus having difficultly in providing acceptable overwrite characteristics.
The conventional recording methods applicable to multispeed recording for optical storage media provide higher power ratio for higher recording linear velocity, the maximum power ratio at a medium linear velocity whereas the minimum power ratio at a low linear velocity or a constant erasing power at all linear velocities. These methods, however, cannot offer sufficient initial recording characteristics or overwrite characteristics because of large erasing power Peh at high linear velocity or cannot offer sufficient overwrite characteristics because of small erasing power Pel at low linear velocity.